1. Field of the Invention
This invention relates to a braking force controller for an electric car. More particularly, it relates to a braking force controller which utilizes the regenerative braking force from a motor mounted in the electric car.
2. Description of the Related Art
When an ABS (Anti-lock Braking System) is practiced in electric cars, it may be possible, in principle, to separately mount an ABS apparatus on a hydraulic brake, of a type similar to that used in ordinary cars, and to stop regenerative braking, using a motor, during the ABS operation. However, according to such an arrangement, energy regeneration to a battery at the time of braking, which constitutes an important feature of electric cars, is not carried out.
Accordingly, various ABSs using regenerative braking by the mounted motor have been attempted, and Japanese Unexamined Patent Publication (Kokai) No. 48-2515, for example, discloses a system for accomplishing anti-lock braking by detecting the speed of the driving wheels, estimating the car speed, and thus the slip ratio, from the wheel speed and controlling the regenerative braking force (first prior art example). Japanese Unexamined Utility Model Publication (Kokai) No. 5-2501 discloses a system for controlling the regenerative braking force by calculating the slip ratio from the difference between the number of revolutions between the driven wheels and non driven wheels (second prior art example).
According to the first prior art example described above, however, it is difficult to accurately estimate the car speed from only the wheel speed, and there is a natural limit to regenerative braking at the suitable slip ratio (approximately 0.2) at which road surface friction becomes maximal. According to the second prior art example, on the other hand, the wheel speed of the follower wheels does not correctly correspond to the car speed when the mechanical braking force on the follower wheels is great, and regenerative braking at the suitable slip ratio is difficult, as well.
In order to solve these problems with the prior art, the present invention is directed to providing a braking force controller for an electric car which can produce suitable ABS regenerative braking by keeping the braking torque of a motor at a maximum.